Chemical dispensers find widespread domestic and industrial application as a convenient means of applying a finite dose of a particular chemical or chemical formulation to a specific target. Typical examples include domestic aerosol containers, industrial paint sprayers, medicament dispensers and agrochemical sprayers.
Medicament dispensers are widely used in the administration of medicines, particularly those for the treatment of respiratory disorders. In dispensing medicaments it is highly desirable that the dose dispensed on each occasion should be consistently at a predetermined level, to ensure that the patient receives the correct dosage. Every effort is therefore made in the design and manufacture of medicament dispensers and their component parts, particularly valve assemblies, to minimise any variations in medicament dosage. However, while great progress has been made in this area in recent years, there is still a risk that on some occasions the requisite dose may not be dispensed. A variety of factors are pertinent in this regard, ranging from lack of uniform dispersal of the formulation to inexperience of the patient in using the device.
One such factor that may occur on occasions, which is of particular relevance to the present invention, is the adherence of medicaments and/or formulation additives to the internal surfaces of the valve and its component parts. This phenomenon is reversible in nature and may lead to greater or lower doses of medicament being dispensed throughout the lifetime of the device. This in turn can result in problems of inconsistent dose administration to the patient and in determining how many effective doses remain within the dispenser. An associated, if minor, problem is that the patient experiences difficulties in manually operating the valve as he perceives a ‘notchiness’ as the valve stem is depressed and released.
Electrostatic interaction between the chemicals comprising the medicament and/or formulation additives and the internal surface of the valve may be a contributing factor which leads to the problem of dose variation described above. As can be seen from the prior art discussed below, the internal surfaces of these valves are often composed of a variety of materials which may favour electrostatic attraction between the medicament and/or formulation additives and the internal surfaces of the valve. Until now, however, the contributory role electrostatic attraction may have in the adhesion of chemicals to the internal surfaces of the valve has not been recognised and no one has addressed the resulting problem of dose variation in the manner described below.
The prior art teaches that a variety of materials may be used in the manufacture of aerosol valve assemblies. Thus U.S. Pat. No. 3,580,431 describes a method of making aerosol valve tip and stem assemblies from hard materials such as polycarbonates, epoxy resins and metals. UK patent application GB 232351 discloses the use of acetal, nylon, polyester or metal in the preparation of annular lips and flanges in metering valves for pressurised dispensing containers. Similarly UK patent application GB 2198117 describes the construction of the valve body, cup and stem of an aerosol metering device using metal components, while European patent application EP 115186 discloses elements of such valves comprised of metals. Furthermore, valve assemblies consisting of stainless steel stems and aluminium ferrules are known in the prior art (e.g. the ‘Spraymiser valve’ from 3M Neotechnic Ltd.).
The present invention teaches that construction of the internal surfaces of the valve and those of the supporting housing, either totally or substantially of a highly conductive material, can improve the problems of chemical dose variation and notchiness discussed above. The use of conductive materials in the construction of these surfaces may significantly reduce the build up of electrostatic charge by facilitating electrical discharge to an earthed point and thus ameliorate the aforementioned problems.